This invention relates to olefin polymerization processes and the resultant polymer products.
Ultra-high molecular weight olefin polymers, such as polyethylene, are useful in many demanding and extremely critical applications, such as human joint replacements, gears, bullet proof vests, skis, and other applications. Since ultra-high molecular weight polymers cannot be pelletized after leaving the reactor, the polymer must be sold as a fluff or a powder. Therefore, particle size and toughness of the resultant polymer is critical.
Many commercial methods are available to produce olefin polymers, such as polyethylene. One of the most economical routes to most commercial grades of olefin polymers is a continuous loop/slurry process with a paraffin diluent wherein the polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent. Unfortunately, most commercially acceptable ultra-high molecular weight polyethylenes traditionally are made using a stirred tank, i.e., batch process, in a heavy hydrocarbon diluent.
Therefore, it is an object of this invention to provide a novel catalyst system which can produce polyethylene.
It is another object of this invention to provide a novel catalyst system which can produce very tough, ultra-high molecular weight polyethylene.
It is still another object of this invention to provide a very tough, ultra-high molecular weight polyethylene.
It is a further object of this invention to provide an improved olefin polymerization process.
It is yet another object of this invention to provide an improved polymerization process for preparing ultra-high molecular weight polyethylene.
In accordance with this invention, a process is provided to polymerize ethylene in a loop/slurry process using a zirconium-containing catalyst system to produce a very tough, ultra-high molecular weight polyethylene.
In accordance with another embodiment of this invention, a very tough, ultra-high molecular weight polyethylene is provided.
In accordance with another embodiment of this invention an exceptionally broad molecular weight distribution polyethylene is provided.
In accordance with still another embodiment of this invention a polymerization process to produce an exceptionally broad molecular weight distribution polyethylene is provided.
Supports of the catalyst system of this invention must be alumina-containing material. As used in this disclosure, the term xe2x80x9csupportxe2x80x9d refers to a carrier for another catalytic component. However, a support is not necessarily an inert material; a support can contribute to catalytic activity and/or catalytic productivity. Furthermore, a support can have an effect on the properties of the resultant polymer produced.
The alumina-containing material used in this invention can contain other ingredients which are present to produce some unrelated result and/or which do adversely affect the quality of the final catalyst system. For example, other metal oxides, such as boria, magnesia, silica, thoria, titania, zirconia, and mixtures thereof, can be present without adverse affects. Preferably, the support is at least 75 weight percent alumina, preferably 85 weight percent alumina, based on the weight of the alumina-containing material, in order to achieve optimum catalyst system quality, as well as improved polymer characteristics. Often, the alumina will comprise some silica.
The alumina-containing material, hereinafter also referred to as xe2x80x9caluminaxe2x80x9d or xe2x80x9cbase aluminaxe2x80x9d, must have a high surface area, large pore volume, and must be calcined prior to use. Usually, the surface area of the alumina, after one hour of calcination at 600xc2x0 C., will be greater than about 200 square meters per gram (m2/g) and preferably within a range of about 200 to about 600 m2/g. Most preferably, the alumina will have a surface area within a range of about 250 to about 500 m2/g for easier catalyst loading, improved productivity, and greater durability. Usually, the pore volume of the alumina will be greater than about 0.5 milliliters per gram (ml/g) and preferably within a range of about 1.0 to about 2.5 ml/g. Most preferably, the alumina will have a pore volume within a range of 1 to 2 ml/g for greater durability.
Exemplary aluminas are commercially available. Preferred commercially available aluminas are commonly referred to as Ketjen B or Ketjen L aluminas. Typical Ketjen B or Ketjen L aluminas, as used in the present invention, will have typical analyses as given in Table A, below.
Prior to treatment with or contacting any additional support components or the active catalytic component, the alumina must be calcined. The alumina is calcined under conditions of temperature and time sufficient to convert substantially all of the alumina hydrate to gamma-alumina and to remove substantially all water. Generally, temperatures within a range of about 300xc2x0 to about 900xc2x0 C., for times within a range of about 1 minute to about 48 hours are sufficient. Temperatures under about 300xc2x0 C. and times of less than about one minute can be insufficient to covert substantially all of the alumina to gamma-alumina. Temperatures of greater than about 900xc2x0 C. and times of greater than about 48 hours do not convert a significantly greater portion of the alumina to gamma-alumina. Preferably, calcination temperatures within a range of about 500xc2x0 to about 800xc2x0 C. and times within a range of about 30 minutes to about 24 hours are employed. Most preferably, temperatures within a range of about 500xc2x0 to about 700xc2x0 C. and times within a range of about 1 hour to about 6 hours are employed. The calcining can be carried out under an oxidizing, reducing, or inert atmosphere; the principal purpose of the atmosphere is to sweep away moisture. However, for ease of use air is the preferred calcination atmosphere. As used in this disclosure, the terms xe2x80x9cgamma-aluminaxe2x80x9d, xe2x80x9ccalcined aluminaxe2x80x9d, and xe2x80x9ccalcined, gamma-aluminaxe2x80x9d are used interchangeably and refer to the calcined alumina, described above.
Optionally, the alumina can be fluorided wherein the alumina support is treated with fluorine-containing compound. Exemplary fluoriding treatments can be found in U.S. Pat. No. 5,171,798 (McDaniel et al.), herein incorporated by reference. The alumina also can be treated with a phosphating agent to provide a phosphated-alumina. Exemplary phosphating methods are described in U.S. Pat. No. 5,001,204 (Klendworth et al.), herein incorporated by reference.
According to one embodiment of this invention, the particle size of the ultra high molecular weight polymer fluff is critical. It has been found that a correct selection of particle size of the catalyst system support particles can control the particle size of the resultant polymer fluff. Usually, catalyst system support particles are within a range of about 1 to about 40 microns, preferably within a range of about 2 to about 20 microns. Most preferably, in order to have an optimally sized polymer product, catalyst support particles are kept within a size range of 4 to 16 microns.
Novel catalyst systems used in the present invention must contain zirconium. Zirconium can be combined with the catalyst system support in accordance with any method know in the art. Preferably, a zirconium halide, such as for example zirconium tetrachloride (ZrCl4), is dissolved in an anhydrous, non-protic, polar solvent. Exemplary solvents include, but are not limited to, tetrahydrofuran, acetonitrile, and mixtures thereof. Alternatively, the zirconium halide can be dissolved in a hydrocarbon such as, for example, toluene, in which about 1 to about 2 moles of an alcohol, such as, for example, butanol, per mole of zirconium has been added. The zirconium halide solution can be mixed slowly, or slurried with, the catalyst system support.
The solvent must be evaporated to dryness under an inert atmosphere to yield a catalyst system. In order to insure complete removal of excess solvent, drying, or curing, of the catalyst system in a fluidized inert atmosphere, under temperatures and times sufficient to remove any excess solvent can be used. Usually, cure temperatures of greater than about 200xc2x0 C. is sufficient. Preferably a cure temperature above about 250xc2x0 C. is used. Most preferably, cure temperatures within a range of 250xc2x0 C. to 600xc2x0 C. is used for best catalyst system activity. The resultant catalyst system can be stored under an inert atmosphere until ready for use.
If a zirconium compound other than a zirconium halide is used, such as a zirconium alkoxide or a zirconyl salt, then the alumina need not necessarily be calcined prior to incorporation of the zirconium. However, it is preferred that the zirconium containing alumina then be given an elevated temperature halide treatment. Such treatments can consist of exposing the material to halide containing vapor, such as carbon tetrachloride, at elevated temperatures. Haliding agents can be any organic or inorganic liquid or vapor containing halide, such as silicon tetrachloride, chloroform, titanium tetrachloride, boron trichloride, hydrogen chloride, silicon hydrotrichloride, etc. Haliding temperatures can be from 200xc2x0 C. to 800xc2x0 C., preferably from 300xc2x0 C. to 600xc2x0 C., most preferably from 300xc2x0 C. to 500xc2x0 C. for best haliding results.
Zirconium usually is present in the catalyst system in an amount within a range of about 0.01 to about 15 weight percent, preferably within a range of about 0.1 to about 10 weight percent, based on the total mass of the catalyst system (support plus zirconium compound). Most preferably, zirconium is present in the catalyst system in an amount within a range of 4 to 8 weight, percent based on the total mass of the catalyst system for best catalyst system activity and productivity, as well as best polymer product particle size.
According to another embodiment of this invention, the inventive, novel zirconium containing catalysts systems can be used in combination with a second catalyst system in the presence of hydrogen to produce an exceptionally broad molecular weight distribution polymer. This second catalyst system contains titanium halide and is commonly referred to as a xe2x80x9cZiegler-Nattaxe2x80x9d catalyst. Commercially available titanium catalyst systems typically comprise complexes of titanium halides and magnesium with organometallic compounds, such as aluminum alkyls. Exemplary Ziegler-Natta, or magnesium/titanium, catalyst systems include, but are not limited to, those disclosed in U.S. Pat. Nos. 4,394,291; 4,326,988 and 4,347,158, herein incorporated by reference.
These two catalyst systems can be combined by any method known in the art. For example they can be premixed prior to being introduced into the reactor. Or preferably the two catalyst systems can be fed to a polymerization reactor independently through two separate feed streams. An aluminum alkyl cocatalyst must be used, and hydrogen must be added to the polymerization reactor. The amount of hydrogen in the reactor can vary from about 0.1 mole percent to about 0.2 mole percent based on the weight of the diluent. Comonomer, such as 1-hexene, also can be added to regulate the density of the polymer if desired.
While not wishing to be bound by theory, it is believed that the novel zirconium containing catalysts of this invention are significantly less sensitive to hydrogen as a molecular weight regulator than are the titanium containing catalysts. Thus, by using both catalyst systems in a single polymerization reactor, with a sufficiently high concentration of hydrogen, so called xe2x80x9cbimodalxe2x80x9d polymers can be produced having a broad molecular weight distribution because the zirconium catalyst system can produce an ultrahigh molecular weight polymer and the titanium catalyst system can produce a low molecular weight polymer.
Catalyst systems of the present invention must be used with a cocatalyst. Cocatalysts useful in the present invention must be an aluminum alkyl cocatalyst, as expressed by the general formulae AlR3, AlR2X, and/or AlRX2, wherein R is an alkyl group having from about 1 to about 12 carbon atoms per alkyl group and X is a halogen atom. Exemplary aluminum alkyl cocatalysts include, but are not limited to triethylaluminum (TEA), triisobutyl aluminum (TIBAL), diethylaluminum chloride (DEAC), ethylaluminum sesquichloride (EASC), and mixtures of two or more thereof. Preferably, the cocatalyst is a trialkyl aluminum cocatalyst, such as TEA, TIBAL and mixtures thereof for best catalyst system activity and reactivity.
The cocatalyst can be combined with the catalyst system in accordance with any method known in the art. Another cocatalyst addition method is to directly add cocatalyst to the reactor either prior to or simultaneously with the novel supported zirconium catalyst system. An additional method of adding cocatalyst is to precontact supported zirconium catalyst with cocatalyst, prior to addition to the reactor.
Generally, the cocatalyst can be present in the reactor in an amount within a range of about 1 to about 500 mg/kg (ppm), based on the weight of diluent, such as isobutane, in the reactor. Preferably, the cocatalyst is present in the reactor in an amount within a range of about 5 to about 100 mg/kg in order to optimize catalyst system activity and productivity. As stated earlier, precontacting catalyst and cocatalyst can occur, but is not required. While not wishing to be bound by theory, it is believed that precontacting catalyst system and cocatalyst can reduce the quantity of cocatalyst used in the reactor.
Polymers produced in accordance with the process of this invention predominately are homopolymers of ethylene or copolymers of ethylene and higher alpha-olefin comonomers. For ultra-high molecular weight (UHMW) polyethylene, trace amounts of comonomers can be present, but comonomers preferably are not present in any significant amount since comonomers can reduce the molecular weight of the desired ultra-high molecular weight polymer product. Preferably, ethylene concentration in the polymerization reactor is within a range of from about 2 weight percent to about 15 weight percent, based on the total liquid contents of the reactor. Most preferably, ethylene concentration in the polymerization reactor is within a range of from about 4 to about 7 weight percent. While ethylene concentration does not significantly affect the molecular weight of the resultant polymer, higher or lower ethylene concentration can effect catalyst activity.
Polymerization of the monomer must be carried out under continuous loop/slurry, also known as particle form, polymerization conditions wherein the reactor temperature is kept below the temperature at which polymer swells. Such polymerization techniques are well known in the art and are disclosed, for instance, in Norwood, U.S. Pat. No. 3,248,179, herein incorporated by reference. A loop/slurry polymerization process is much more preferred than a stirred tank reactor because a loop reactor has a greater heat transfer surface, much more versatility for plant operation, usually less polymer swelling during polymerization, and diluent can be flashed off, eliminating the necessity of separating polymer product from solvent.
To produce UHMW polyethylene, the temperature of the polymerization reactor, or reaction zone, according to this invention, is critical and must be kept within a range of about 170xc2x0 F. to about 230xc2x0 F. (76xc2x0 C. to 110xc2x0 C.), or about 170xc2x0 F. to about 220xc2x0 F. (76xc2x0 C. to 105xc2x0 C.). preferably within a range of about 180xc2x0 F. to about 210xc2x0 F. (82xc2x0 C. to 99xc2x0 C.). Most preferably, the reaction zone temperature is within a range of 190xc2x0 F. to 200xc2x0 F. (87xc2x0 C. to 94xc2x0 C.). The temperature range is critical in order to produce an ultra-high molecular weight polyethylene. Too high of a reactor temperature can produce a polymer with too low of a molecular weight; too low of a reactor temperature can make the polymerization process inoperable because a lower reactor temperature can be difficult to control due to the exothermic polymerization reaction, flashing off reactor diluent can be difficult, and a lower reactor temperature can produce a polymer with a commercially unacceptable (too low) molecular weight. To produce other types of polymers, higher reactor temperatures can be used.
The loop/slurry process used in this invention must be carried out in an inert diluent (medium) selected from the group consisting of hydrocarbons having three and four carbon atoms per molecule. Exemplary diluents include, but are not limited, to propane, n-butane, isobutane, and mixtures thereof. Diluents having more or less than three or four carbon atoms per molecule can be difficult to separate from the polymer product during the polymer recovery process. Isobutane is the most preferred diluent due to low cost and ease of use.
Pressures in the loop/slurry process can vary from about 110 to about 1000 psia (0.76-4.8 MPa) or higher, preferably 500 to 700 psia. The catalyst system is kept in suspension and is contacted with ethylene at a pressure sufficient to maintain the diluent and at least a portion of the ethylene in a liquid phase. The reactor diluent and temperature thus are selected such that the polymer is produced and recovered as solid particles. Catalyst system concentrations in the reactor can be such that the catalyst system content ranges from 0.0001 to about 0.1 weight percent based, on the weight of the reactor contents.
To produce UHMW polyethylene, hydrogen never is added to the polymerization reactor because hydrogen has too great of an effect on the molecular weight of the resultant polymer. For other types of polymers, hydrogen can be added to the reactor to control molecular weight.
UHMW polymers produced in accordance with this invention are considered homopolymers of ethylene, even though trace, insignificant amounts of comonomers can be present in the resultant polymer. Polymers produced according to this invention have a weight average molecular weight (Mw), generally above one million (1,000,000), are considered ultra-high molecular weight polymers. Preferably, polymers produced in accordance with this invention have a weight average molecular weight of greater than about two million (2,000,000) and most preferably, within a range of greater than or equal to about 2,500,000 up to about 10,000,000.
Since the molecular weight of these polymers is so high, the polymers exhibit a value of zero (0) for both the melt index (MI) and high load melt index (HLMI). The inherent viscosity (IV) of the polymers generally is greater than about 19, preferably within a range of about 20 to about 30. Most preferably, the polymers will have an IV within a range of 22 to 28.
The density of these novel polymers usually is within a range of about 0.92 g/cc to about 0.94 g/cc, preferably from about 0.925 to about 0.936 g/cc. Most preferably, polymer density is within a range of about 0.927 to about 0.933 g/cc.
Another critical, defining physical characteristic of these polymers is the fluff, or powder, size. Usually, the particle size is less than about 400 microns (40 mesh), preferably within a range of about 400 microns to about 40 microns (300 mesh). Most preferably, the particle size is within a range of about 50 to about 400 microns. Particle sizes of larger that about 400 microns often can appear in the in the finished product as a flaw, or a white patch. While not wishing to be bound by theory, it is believed that this defect appears because the particles are not molded by typical methods in the art, but are merely fused together by compression. Particles that are too fine, or small, can inhibit transport of the polymer fluff (powder) through conveyor blowers because the fine particles can cling to walls by static and can plug downstream filters due to blowover.
Polymers produced according to this invention must be very tough, as evidenced by the sand wheel abrasion test, tensile strength, elongation, flexural modulus, hardness and Izod impact strength.
High bulk density also is important because bulk density is related to the amount of compression of the polymer during fusion. A low bulk density can inhibit and slow down processing rates. Generally, polymers produced in accordance with this invention have a bulk density of greater than about 0.25 g/cc, preferably, greater than about 0.3 g/cc. Most preferably, polymer bulk density is within a range of 0.35 to 1 g/cc.
A further understanding of the present invention and its advantages are provided by reference to the following examples.
According to another embodiment of this invention, novel zirconium containing catalyst systems of this invention can be used in combination with a second catalyst system in the presence of hydrogen to produce an exceptionally broad molecular weight distribution polymer. This second catalyst system comprises a titanium halide and a magnesium compound and is commonly referred to as a xe2x80x9cZiegler-Nattaxe2x80x9d catalyst system. Commercially available titanium catalyst systems typically comprise complexes of titanium halides with organometallic compounds, such as, for example, aluminum alkyls. Exemplary magnesium/titanium catalyst systems include, but are not limited to, those disclosed in U.S. Pat. Nos. 4,394,291; 4,326,988; and 4,347,158, herein incorporated by reference.
The two catalyst systems can be combined by any method known in the art. For example, they can be premixed prior to being introduced into a polymerization reactor. Or preferably, the two catalyst systems are fed to the reactor independently through two separate feeders. An aluminum alkyl cocatalyst must be used, and also hydrogen must be added to the reactor. The amount of hydrogen in the reactor can vary from 0.1 mole percent to 2.0 mole percent based on the weight of the diluent. Comonomer, such as 1-hexene, also can be added to regulate the density of the polymer if desired.
While not wishing to be bound by theory, it is believed that the novel zirconium containing catalyst system of this invention are significantly less sensitive to hydrogen as a molecular weight regulator than are the titanium-containing catalysts. Thus, by using both catalyst systems in a single polymerization reactor, with a sufficiently high concentration of hydrogen, so called xe2x80x9cbimodalxe2x80x9d polymers are produced having a broad molecular weight distribution because the novel zirconium catalyst system can produce an ultrahigh molecular weight polymer portion and the titanium catalyst system can produce a low molecular weight portion polymer.